Convert types.d.mts from ambient module declaration to module augmentation which enables VS Code to handle @type {import("markdownlint").Rule} (https://www.typescriptlang.org/docs/handbook/modules/reference.html#ambient-modules), fix broken markdownlint @typedefs in helpers/*.cjs.

lib/types.d.mts

@@ -1,3 +1,4 @@
+export {};
 declare module "markdownlint" {
   export * from "./exports.mjs";
 }